(1) Field of the Invention
This invention relates to a method manufacturing a semiconductor device, and more particularly to a method of forming a twin-tub structure for a CMOS (Complementary Metal Oxide Semiconductor) device.
(2) Description of the Related Art
In the formation of CMOS field effect transistor (FET) integrated circuits, in a semiconductor substrate, it is necessary to provide regions in the substrate tailored for each type of transistor. For example, in the case of CMOS FET's, the P-channel devices must be located in regions having an N type doping, and the N-channel devices must be located in regions having a P type doping. There are three approaches to forming the two different substrate dopings: (1) P tub or well, (2) N tub or well, and (3) twin tub or well processes.
The most preferred of the three processes is the twin tub or well process, particularly for very high density integrated circuits with feature sizes of one micrometer or below. The primary advantage of the twin tub is that the process allows the doping profile in each tub or well region to be independently tailored for optimum device characteristics. The substrate itself can be either lightly doped P or N type.
Workers in the art have been active in developing twin tub processes. One example is Parrillo et al U.S. Pat. No. 4,435,896. An N tub is formed in one portion of a lightly doped silicon substrate by implantation while masking the future P tub region with an oxide/nitride mask. The mask is removed, and a thick oxide is grown over the N tub, which acts as a mask for the implant to form the P tub, which is self-aligned with the N tub. The tubs are driven in by a diffusion step, and thick field oxide regions formed using the LOCOS (Local Oxidation of Silicon) technique. However, the LOCOS process has many furnace processes such as formation of a buffer oxide, nitride deposition, and field oxidation. This increases machine loading and process cycle time. For example, formation of a 300 Angstrom buffer oxide requires about 30 minutes, a 1500 Angstrom nitride deposited by Low Pressure Chemical Vapor Deposition (LPCVD) takes approximately 90 minutes, and a 5000 Angstrom thick field oxide requires between three and eight hours.